A conventional valve device of the foregoing type is arranged such that a motor shaft is axially moved by the rotation of a motor rotor to cause the lower end of the motor shaft to abut against the upper end of a valve stem and a valve element is detached from a valve seat with the following movement of the motor shaft. Thus, the conventional valve device is constituted of a motor shaft and a valve stem which are formed independently from each other, and the shock caused by the collision of a valve element with a valve seat at the time of closing of the valve has been reduced during the time interval in which the motor shaft is being moved in a space leading to a motor stopper by the inertial force of the rotor after the motor shaft is separated from the valve stem.
Otherwise, there is a valve device where a motor shaft and a valve stem have an integrally assembled structure as disclosed in Patent Document 1; however, the valve device is one where a space is provided at both sides of a male screw and a female screw in order to restrict the rotation of a rotor at the time of fully closing of the valve.
Patent Document 1: Japanese Patent Gazette No. 3719915
However, in the conventional valve device constituted of a motor shaft and a valve stem which are independently formed, the motor shaft and the valve stem are separated from each other at the time of undershooting in the valve closing operation, at the time of initialization (operation for studying the initial position), or at the time of seizure of the valve stem, and therefore, an error may arise between a valve position and the output of a sensor detecting the valve position. As a result, there is a problem that there may arise a situation where the valve position cannot be accurately monitored in the engine.
Further, Patent Document 1 does not disclose the detection of the valve position through the valve stem.
The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a valve device such that detection accuracy of a valve position is improved and that pre-load measures to be required at the time of fully closing of the valve is eliminated.